The proposed research consists of three separate but related projects. In the first it will be attempted to discover how general is the finding that a major sex-linked recessive gene is involved in spatial ability and to hopefully clarify the structure of inter- relationships between spatial factors. The second study will focus on the degree to which assortative mating occurs for various cognitive abilities and attempt to elucidate the role of socio-economic- educational factors in assortative mating. In the third project a series of mathematical and statistical investigations of models for the development of cognitive abilities, involving correlated genotypic and environmental effects and assortative mating will be undertaken.